In-line optical fiber diffraction gratings are known. One method writing an in-line grating into optical fiber comprises interfering two beams of actinic (typically UV) radiation in a portion of the fiber. The two beams are incident on the fiber in a transverse direction, the angle between the two beams (and the wavelength of the radiation) defining the grating spacing. Typically fiber gratings are formed in germanium-doped fiber and the sensitivity of the germanium doped fiber to actinic radiation is enhanced by a thermal hydrogen treatment. The hydrogen loading is done at a pre-determined pressure and temperature. This method can be used to write gratings in fiber or planar silica waveguides and to write a channel into the core layer of planar optical waveguides. The waveguide glass, for example a silica glass, must contain a photorefractive-sensitive component, such as germanium dioxide.
U.S. Pat. No. 5,235,659 and related U.S. Pat. No. 5, 287, 427 disclose that large normalized refractive index changes can be obtained in silica-based optical waveguides (fiber or planar waveguides) by a treatment that comprises exposing at least a portion of the waveguide at a temperature of at most 250.degree. C. to hydrogen at moderate pressures and irradiating a part of the exposed portion with actinic radiation. In particular, there is disclosed a method for modifying the refractive index of planar waveguides (col. 5, line 34 to col. 6, line 9). A layered structure comprising, for example, a silicon substrate on which is formed a lower cladding layer of vitreous silica, an intermediate (core) germanosilicate layer, and an upper cladding layer of vitreous silica is formed. This structure has waveguiding properties in the vertical plane of the core layer but does not confine the radiation in the horizontal plane of the core layer. Horizontal confinement is achieved by loading the structure with hydrogen, irradiating the structure with focused actinic radiation normal to the layered structure, and moving the beam over the structure in a predetermined manner. The refractive index in the irradiated region of the germanosilicate core layer is raised above the adjacent lateral portions of the core layer, providing lateral guiding. The refractive index of the cladding layers remains essentially unchanged.
In such a structure, as described in U.S. Pat. 5,235,659, the writing is done in a layered structure already presenting planar waveguide properties because vertical confinement is already provided by the fact that the core layer containing germanium dopant has a refractive index above the refractive indices of the underclad and overclad layers which lack the dopant. After the writing is done the refractive index of the irradiated region of the core layer is further increased. The difference in refractive index between the written region of the core and the cladding layers above and below will be greater than the difference in refractive index between the written region of the core and the adjacent lateral regions of the core layer which contain dopant. A disadvantage of such a planar waveguide is that it is an unsymmetrical waveguide, i.e. the difference in refractive index between the written region and adjacent regions is not the same in all directions. In such a structure the difference in index is larger between the channel waveguide and the surrounding structure in the vertical dimension than in the horizontal one. Therefore the mode is less guided horizontally and expands in this direction.
Unsymmetrical waveguides are disadvantageous because of the asymmetric mode field propagated in such a structure. Consequences include an increase in the coupling losses with single mode fiber and a large polarization dependence. The mode field of the unsymmetrical waveguide can be made circularly symmetric by making the waveguide cross-section rectangular, for example, taller than it is wide. This approach will overcome the problem of higher coupling losses. However, this approach still suffers from the fact that coupling will occur between the channel waveguide and the planar or "slab" waveguide. This coupling will result in consmission losses at selected wavelengths corresponding to the equalization of the propagation constant of the eigenmode of the channel waveguide to those (several) of the planar waveguide. The invention for a symmetrical planar waveguide has no "slab" waveguide, so these losses will not occur.
It is an object of this invention to provide a non-waveguiding structure for use in making waveguides with symmetrical waveguide properties.